There has been a surge of interest in Non-Volatile Memory (NVM) in recent years. With many advantages, such as density and power consumption, NVM is carving out a place in the memory hierarchy and may eventually change our view of computer architecture. Many NVMs have emerged, such as Magnetoresistive random access memory (MRAM), Phase Change random access memory (PCM), Resistive random access memory (ReRAM), and Ferroelectric random access memory (FeRAM), each with its own peculiar properties and specific challenges. The scientific community has carried out a substantial amount of work on integrating those technologies in the memory hierarchy. As many companies are announcing the imminent mass production of NVMs, we think that it is time to have a step back and discuss the body of literature related to NVM integration. This article surveys state-of-the-art work on integrating NVM into the memory hierarchy. Specially, we introduce the four types of NVM, namely, MRAM, PCM, ReRAM, and FeRAM, and investigate different ways of integrating them into the memory hierarchy from the horizontal or vertical perspectives. Here, horizontal integration means that the new memory is placed at the same level as an existing one, while vertical integration means that the new memory is interleaved between two existing levels. In addition, we describe challenges and opportunities with each NVM technique.

Internet of Things (IoT) will be one of the driving application for digital data generation in the next years as more than 50 billions of objects will be connected by 2020. IoT data can be processed and used by different devices spread all over the network. The traditional way of centralizing data processing in the Cloud can hardly scale because it cannot satisfy many of the latency critical IoT applications. In addition, it generates a too high network traffic when the number of objects and services increase. Fog infrastructure provides a beginning of an answer to such an issue. In this paper, we present a data placement strategy for Fog infrastructures called iFogStor. The objective of iFogStor is to take profit of the heterogeneity and location of Fog nodes to reduce the overall latency of storing and retrieving data in a Fog. We formulated the data placement problem as a Generalized Assignment Problem (GAP) and proposed two ways to solve it: 1) an exact solution using integer programming and 2) a heuristic one based on geographical zoning to reduce the solving time. Both solutions proved very good performance as they reduced the latency by more than 86% as compared to a Cloud based solution and by 60% as compared to a naive Fog solution. Using geographical zoning heuristic can allow solving problems with large number of Fog nodes efficiently and in a couple of seconds making iFogStor feasible in runtime and scalable.

Fog computing consists in extending Cloud services down to the network edge by using resources such as base stations, routers and switches. It presents a dense, heterogeneous and geo-distributed infrastructure which pushes to investigate how data are placed within this infrastructure in order to minimize service latency, network utilization and energy consumption. iFogSim is a Fog and IoT environments simulator dedicated to manage IoT services in a Fog infrastructure. In this paper, we present an extension to iFogSim to be able to model and simulate scenarios with strategies aiming to optimize data placement in Fog and IoT contexts. Data placement problem is NP-Hard due to the large number of Fog nodes and the high amount of data to be placed. Thus, we added a support to divide and conquer strategies to subdivide the issued infrastructure into several parts hence reducing the data placement computing time. Moreover, the extension involves a generic smart city scenario with different workloads making it possible for the users to investigate the behavior of their strategies using various workloads. In order to optimize the execution time of the simulations, we parallelized the Floyd-Warshall algorithm. This algorithm is used in iFogSim to compute all shortest paths between nodes in order to simulate data transmission. We have evaluated this extension using the proposed smart city scenario with various infrastructure configurations. The experiments show that our extension has a small overhead in terms of simulation time and memory utilization.

Traditional Linux prefetching algorithms were based on spatial locality of I/O workloads and performance model of hard disk drives. From the applicative point of view, current data-intensive applications I/O workloads are turning towards more random patterns while from the storage device perspective, flash based storage devices present a different performance model than HDDs. In this work, we present a new prefetching mechanism named Lynx. Lynx aims to adapt and/or complement the Linux read-ahead prefetching system for both SSD performance model and new applications needs. Lynx uses a simple machine learning system based on Markov chains. The learning phase detects I/O workload patterns and computes the transition probabilities between file pages. The prediction phase prefetchs predicted file pages with the resulting Markov statemachine. We have implemented our solution and integrated it into the Linux kernel. We experimented our solution using the TPCH benchmark. The results show that Lynx divides the number of page cache misses (major page faults) by 2 on average and thus reduces TPC-H queries execution time by 50% as compared to traditional Linux read-ahead.